In the deposition of metals without an outside current, such as in the chemical deposition of copper from corresponding electrolytes without an outside current, a reducing agent is added to the electrolyte, which agent as an interior voltage source makes possible the deposition of the metal.
The basic principle of metal deposition without an outside current will be explained here with the example of a copper electrolyte.
As a rule, electrolytes for chemical copper deposition without an outside current contain complex- or chelate-bound copper ions, such as copper tartrate complexes or copper-EDTA chelates. Formaldehyde or a comparable reducing agent, which, as the result of an oxidation reaction to the formate or to the corresponding anion, provides the electrons needed for the reduction of the copper, is used, as a rule, as the reducing agent.
Formaldehyde, however, is able to act as a sufficiently strong reducing agent on divalent copper ions, as they are used, as a rule, in electrolytes for the deposition of copper without an outside current, and to make possible a metal deposition, only in a highly alkaline pH range such as between about pH 11 and about pH 14. From this, it follows that the copper ions present in the electrolyte are so strongly complexed or chelated that they cannot form hard-to-dissolve metal hydroxides.
Moreover, copper is introduced into the electrolyte, as a rule, in the form of sulfates. As a consequence of the reaction of divalent copper ions to elementary copper, the electrolyte is enriched with sulfate anions. This sulfate anions enrichment, produced by the oxidation of the formaldehyde in combination with the concentration increase of formate anions, leads to a lowering of the pH value. In order to continue to hold the electrolyte in a workable pH range, alkali hydroxides, such as sodium hydroxide, are added. Moreover, the consumed quantities of copper sulfate and formaldehyde are subsequently metered to the electrolyte. As a result of the foregoing, the chemical and physical characteristics of the electrolyte therefore change, which leads to a limited durability and applicability of the electrolyte.
Nickel baths without a current work mostly in an acidic pH range. There, bath maintenance by means of electrodialysis is already known from documents EP 1 239 057 A1, DE 198 49 278 C1, and EP 0 787 829 A1. The process described there and the combination of methods and membranes cannot be used, however, for copper without a current, or for others in alkaline metallization baths working without a current.